This invention relates to artificial denture teeth and in particular to artificial posterior teeth. A principal objective in artificial teeth of this type is to provide a balanced occlusion of opposing maxillary and mandibular teeth. A "balanced occlusion" may be defined as a harmonious relation of the occluding surfaces in centric and eccentric positions within the functional range of the mandible. When a balanced occlusion is achieved, there is simultaneous contact of opposing maxillary and mandibular teeth on both sides of the mouth as the mandible moves within its functional range. This relationship ensures adequate surface area of occlusal contact so as to promote comfort, function and esthetics. In the absence of a balanced occlusion there is a likelihood of deflective occlusal contacts between opposing teeth. Some of the symptoms of this problem include cheek biting, tissue soreness, a lack of chewing efficiency, and unpleasing esthetic effects. Also, this greatly reduces the lateral and horizontal forces applied to the supporting tissues through the denture bases.
The artificial teeth described in the prior art are typically formed of porcelain or plastic. The prior art teeth have been satisfactory from the standpoint of size and shape; in general, it has been possible to closely conform artificial teeth to the sizes and shapes of natural teeth. It has also been possible to achieve a balanced occlusion with the prior art teeth. However, this has required extensive grinding of the teeth to establish the needed tooth contacts for mechanical harmony. Grinding is performed either prior to or following processing of the dentures, and often requires a great deal of effort on the part of the dentist or technician.
The problems of grinding to achieve balanced occlusion are heightened when the oral conditions or "patient factors" of a particular patient do not fit into the circumstances dictated by the design of the denture tooth. Examples of patient factors which normally are not accounted for in the prior art teeth include poor neuromuscular control of jaw movements, discrepancies in the anteroposterior and mediolateral jaw relationships and the need of the patient to look natural and present a pleasant appearance. When patient factors such as these are present, extensive occlusal modification is necessary or balanced occlusion is unattainable. None of the prior art teeth are capable of providing a balanced occlusion universally to every patient without extensive grinding.
Another problem encountered by the prior art teeth is that, even if a balanced occlusion is initially achieved, changes in the patient's stomatognathic system over time can alter tooth contact positions and destroy the balanced relationship. Physiological changes normally occur in the denture supporting tissues as a function of patient aging. As these tissues undergo change, the denture bases assume new positions. This changes the contact relationship of the upper and lower teeth, so that an occlusion that was initially balanced may become deflective. The resulting deflective contacts may accelerate the changes in supporting tissues and may even induce pathological changes in the tissues.
It is an object of the present invention, therefore, to provide an artificial posterior tooth which would permit the formation of a balanced occlusion without extensive grinding. Another object is to provide an artificial posterior tooth which would permit the formation of balanced occlusion in all patients, regardless of variations in individual patient factors, without extensive grinding. Still, another object is to provide an artifical posterior tooth which is adaptable over time to changes in the patient's stomatognathic system to ensure a continuously balanced occlusion.
The artificial posterior tooth of the present invention consists of a body formed of a plurality of resin materials varying in hardness, wear resistance, and strength. The body is shaped to conform generally to the shape of a natural tooth. The resin materials are selectively distributed throughout the tooth body to produce an uneven wear pattern which is consistent with the changes expected in the individual patient's stomatognathic system.
In a preferred embodiment of a denture system produced according to this invention, each pair of opposing maxillary and mandibular teeth is constructed of three resins. A first resin is used to form the lingual cusp(s) (a premolar has a single lingual cusp, while a molar has two, known as the mesiolingual and distolingual cusps) and an adjacent portion of the buccal cusp(s) of the maxillary tooth. A second resin is used to form the remainder of the maxillary tooth. The second resin is also used to form the lingual cusp(s) and an adjacent portion of the buccal cusp(s) of the mandibular tooth. A third resin is used to form the remainder of the mandibular tooth. The first resin has strength, wear resistance, and hardness characteristics which are greater than those of the second resin, which in turn has greater strength, wear resistance, and hardness than the third resin. Examples of resins having acceptable characteristics include the following:
Filled and unfilled dental resins which contain up to 80% of the inorganic fillers, for example, glass beads, rods, quartz, or lithium aluminum silicate. The particular filler or fillers used will determine the hardness of the finished resin. The filler particles are coated with a coupling agent, such as an appropriate silicone, to provide an adhesive bond between the filler and the resin. The matrix resin is a reaction product of the epoxy resin (glycidye acrylate). The reaction product is thinned with methyl methacrylate or some other comonomer of low viscosity.
In the embodiment described above, the areas of wear will be the occluding surfaces of the mandibular tooth (resins #2 and 3) as it wears against the maxillary occluding surface (resin #1) in various jaw movements. The least wear will occur in the area of greatest contact, that is, where the maxillary lingual cusp (resin #1) contacts the opposing occlusal surface of the mandibular tooth (resin #2). Thus, wear will occur where it is needed to prevent development of deflective contacts but not where it would detract from mechanical effectiveness.
It is to be understood that the resins listed above are for illustrative purposes only, and the invention is not limited to these resins or to the embodiment described above. Whichever resins are employed may be selectively distributed throughout the tooth body in any manner which will achieve the selective wear characteristics necessary to ensure a continuously balanced occlusion.
The invention thus provides an artificial posterior tooth with selective wear characteristics which inhibit the development of deflective occlusal contacts and promote the development of a balanced occlusion. The tooth can therefore be used to achieve a balanced occlusion for all patients, regardless of individual oral conditions. The distribution and composition of the resins can be varied from patient to patient to produce a wear pattern that ensures balanced occlusion despite temporal physiological changes in the individual patient's stomatognathic system. Thus, the tooth of this invention enables a more esthetically pleasing appearance, functions more harmoniously and is more comfortable and physiologically beneficial than teeth described in the prior art.